In order to quantify the potential methane hazard in underground coal mines, it is necessary to determine the in-seam methane content and an expected gas release rate during production. Various methods for estimating this have been proposed, ranging from empirical formulae to direct measurement of the methane released from coal samples in the form of drill chippings or cores. These methods generally suffer similar drawbacks, i.e. they are site-specific, have long time delays before data become available, use small sample sizes and/or inaccurate estimation methods, often involve cumbersome test and post-test procedures, etc. Despite their shortcomings, these are the only methods currently available to the industry. It was decided to investigate alternative methods that would be inexpensive, easy to use by mine personnel, robust enough for use in a production section, and provide information in as short a time as possible. To overcome the above drawbacks, a new approach was explored that would eliminate as far as possible the current drawbacks.
Initial investigations were focused on a theoretical gas release rate model which postulates that five parameters govern the rate of gas release from cut coal. The model was modified to simulate pressure build-up due to gas release into a hermetically sealed environment. The test was performed physically by sealing a cut coal sample in a canister and measuring the pressure build-up due to gas release. The theoretical model’s pressure vs time pressure curve was then ‘adjusted’ by varying the identified governing parameters to fit the measured curve. This will uniquely describe the parameters governing gas release, thus allowing the theoretical model to be used to calculate the gas release rate and content for the coal seam under ambient conditions. Although the actual and theoretical pressure curves were similar in shape, it proved very difficult to refine the gas release model sufficiently to give a unique set of governing parameters as convergence of the solution could not be found.
As the hardware for the experiment had already been built, it was decided to investigate the option of using the Ideal Gas Law to determine the rate of gas release from the cut coal for the first 30 minutes after it has been cut from the face. This will give a good indication of the gas generated during production at the face, which is a high-risk area with regard to the ignition of gas explosions.
The results obtained are encouraging. Due to the nature of the system, it gives accurate results, is easier to operate than current systems as it is semi-automated, has self-diagnostic abilities to increase confidence, and the quality of the data allows parameters to be developed that will give advance warning of increasing gas release levels. The system data can also be incorporated into current gas release rate databases or systems.
This report describes the process followed and the results obtained.